Common Lisp the Language, 2nd Edition
An object that has standard-class as its metaclass has zero or more named slots. The slots of an object are determined by the class of the object. Each slot can hold one value. The name of a slot is a symbol that is syntactically valid for use as a variable name.
When a slot does not have a value, the slot is said to be unbound. When an unbound slot is read, the generic function slot-unbound is invoked. The system-supplied primary method for slot-unbound signals an error.
The default initial value form for a slot is defined by the :initform slot option. When the :initform form is used to supply a value, it is evaluated in the lexical environment in which the defclass form was evaluated. The :initform along with the lexical environment in which the defclass form was evaluated is called a captured :initform. See section 28.1.9.
A local slot is defined to be a slot that is visible to exactly one instance, namely the one in which the slot is allocated. A shared slot is defined to be a slot that is visible to more than one instance of a given class and its subclasses.
A class is said to define a slot with a given name when the defclass form for that class contains a slot specifier with that name. Defining a local slot does not immediately create a slot; it causes a slot to be created each time an instance of the class is created. Defining a shared slot immediately creates a slot.
The :allocation slot option to defclass controls the kind of slot that is defined. If the value of the :allocation slot option is :instance, a local slot is created. If the value of :allocation is :class, a shared slot is created.
A slot is said to be accessible in an instance of a class if
the slot is defined by the class of the instance or is inherited from
a superclass of that class. At most one slot of a given name can be
accessible in an instance. A shared slot defined by a class is
accessible in all instances of that class. A detailed explanation of
the inheritance of slots is given in